Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
A typical cellular wireless network includes a number of base stations each radiating to define a respective coverage area in which user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In particular, each coverage area may operate on one or more carriers each defining a respective frequency bandwidth of coverage. In turn, each base station may be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the network may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
Further, a cellular wireless network may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink or forward link and communications from the UEs to the base stations defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Orthogonal Frequency Division Multiple Access (OFDMA (e.g., Long Term Evolution (LTE) and Wireless Interoperability for Microwave Access (WiMAX)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between coverage areas, and other functions related to air interface communication.
Depending on the air interface protocol and other factors, a coverage area may operate using a frequency division duplex (FDD) arrangement or may operate using a time division duplex (TDD) arrangement. In an FDD arrangement, different carrier frequencies or ranges of frequency are used for the downlink than for the uplink. With this FDD arrangement, an FDD carrier may therefore include a pair of frequency channels with a first channel being used for downlink communication and a second channel being used for uplink communication. Whereas, in a TDD arrangement, the same carrier frequency or range of frequency is used for the downlink and uplink and is allocated over time among downlink and uplink communications. With this TDD arrangement, a TDD carrier may therefore include a single frequency channel divided over time into segments for downlink communication and other segments for uplink communication. A recent version of the LTE standard of the Universal Mobile Telecommunications System (UMTS) supports both the TDD arrangement and the FDD arrangement.
Furthermore, in accordance with the recent version of the LTE standard, each coverage area of a base station may operate on one or more carriers spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, with each carrier being divided primarily into subcarriers spaced apart from each other by 15 kHz. The air interface is then divided over time into a continuum of 10-millisecond frames, with each frame being further divided into ten 1-millisecond subframes or transmission time intervals (TTIs) that are in turn each divided into two 0.5-millisecond segments. And each 0.5 millisecond segment or in each 1 millisecond TTI, the air interface is then considered to define a number of 12-subcarrier wide “resource blocks” cooperatively spanning the frequency bandwidth (i.e., as many as would fit in the given frequency bandwidth). In addition, each resource block is divided over time into symbol segments of 67 μs each, with each symbol segment spanning the 12-subcarriers of the resource block and thus supporting transmission of symbols in “resource elements.”
The LTE air interface then defines various channels made up of certain ones of these resource blocks and resource elements. For instance, on the downlink, certain resource elements across the bandwidth are reserved to define a physical downlink control channel (PDCCH) for carrying control signaling from the base station to UEs, and other resource elements are reserved to define a physical downlink shared channel (PDSCH) for carrying bearer data transmissions from the base station to UEs. Likewise, on the uplink, certain resource elements across the bandwidth are reserved to define a physical uplink control channel (PUCCH) for carrying control signaling from UEs to the base station, and other resource elements are reserved to define a physical uplink shared channel (PUSCH) for carrying bearer data transmissions from UEs to the base station.
In a system arranged as described above, when a UE enters into coverage of a base station, the UE may engage in attach signaling with the base station, by which the UE would register to be served by the base station on a particular carrier. Through the attach process and/or subsequently, the base station and supporting LTE network infrastructure may establish for the UE one or more bearers, essentially defining logical tunnels for carrying bearer data between the UE and a transport network such as the Internet.
Once attached with the base station, a UE may then operate in a “connected” mode in which the base station may schedule data communication to and from the UE on the UE's established bearer(s). In particular, when a UE has data to transmit to the base station, the UE may transmit a scheduling request to the base station, and the base station may responsively allocate one or more upcoming resource blocks on the PUSCH to carry that bearer traffic and transmit on the PDCCH to the UE a downlink control information (DCI) message that directs the UE to transmit the bearer traffic in the allocated resource blocks, and the UE may then do so. Likewise, when the base station has bearer traffic to transmit to the UE, the base station may allocate PDSCH resource blocks to carry that bearer traffic and may transmit on the PDCCH to the UE a DCI message that directs the UE to receive the bearer traffic in the allocated resource blocks, and the base station may thus transmit the bearer traffic in the allocated resource blocks to the UE. With this arrangement, when a UE is attached with a base station on a particular carrier, the base station provides DCIs to the UE on the PDCCH of that particular carrier and schedules downlink communication of bearer data to the UE on the PDSCH of that particular carrier. Moreover, LTE also supports uplink control signaling on the PUCCH using uplink control information (UCI) messages. UCI messages can carry scheduling requests from UEs, requesting the base station to allocate PUSCH resource blocks for uplink bearer data communication.
In another arrangement, a revision of LTE known as LTE-Advanced may permit a base station to serve a UE with “carrier aggregation,” by which the base station schedules bearer communication with a UE on multiple carriers at a time. With carrier aggregation, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands can be aggregated to increase the bandwidth available to the UE. Currently, the maximum bandwidth for a data transaction between a base station and a UE using a single carrier is 20 MHz. Using carrier aggregation, a base station may increase the maximum bandwidth to up to 100 MHz by aggregating up to five carriers.
When carriers are aggregated, each carrier may be referred to as a component carrier. Of the component carriers, one may be deemed a primary component carrier or primary cell (PCell) on which the base station serves the UE, and each other component carrier may be deemed to be a secondary component carrier or secondary cell (SCell) on which the base station serves the UE. In particular, the primary carrier may be the carrier on which the UE is attached with the serving base station and may thus carry control signaling (such as scheduling requests and DCI messages) between the base station and the UE, in addition to carrying scheduled data transmissions between the base station and the UE. Each added secondary carrier may then function to increase the total bandwidth on which the base station serves the UE with scheduled data transmissions.
Generally, in order to change a primary carrier on which the base station serves the UE, the serving base station may engage in handover processing. This may specifically involve detaching the UE from the primary carrier on which the UE is attached with the serving base station and then engaging in signaling by which the UE would register to be served (e.g., by the same base station or by a different base station) on a new primary carrier. Whereas, in order to change a secondary carrier on which the base station serves the UE, the serving base station may simply remove the secondary carrier from the carrier aggregation service and may add a new secondary carrier to the carrier aggregation service without engaging in handover processing.